Alginic acid or a derivative thereof (hereinafter, sometimes collectively referred to as “alginic acid”) has property of forming a very viscous and homogeneous solution when dissolved in an aqueous solution, and is used in a product for which such viscous property is demanded in terms of operability and the like, for example, a food such as jam, sauce, or mayonnaise or an ophthalmic composition such as an eye lotion or a contact lens care solution. Further, the alginic acid or the derivative thereof has property of gelling upon contact with a polyvalent metal ion such as a calcium ion, and hence is generally used as a dental impression material, in particular.
However, it is known that such alginic acid exhibits a time-dependent decrease in viscosity in an aqueous solution. Therefore, in an alginic acid aqueous solution or an alginic acid-containing aqueous composition obtained by adding a filler thereto, its viscosity cannot be retained stably over a long period of time, which is a major factor for deterioration in handleability of the composition. In view of the foregoing, in the alginic acid-containing aqueous composition, it has been a big problem to retain the viscosity stably over a long period of time, from the viewpoint of improving storage stability of the composition.
In order to overcome the problem, there have been developed methods of suppressing a decrease in viscosity of an alginic acid-containing aqueous composition by blending an additive in the composition (Patent Literatures 1 to 4). However, all of the methods involve the following problems: there is a limitation on an applicable composition; and the blending of the additive causes a decrease in initial viscosity of the composition, with the result that a desired initial viscosity cannot be obtained.
Meanwhile, a pasty dental alginate impression material blended with an organic hydroxy compound typified by a saccharide has been developed (Patent Literatures 5 and 6). The organic hydroxy compound used in the material is blended in order to improve tooth release property and tray retention property, and one satisfying the following mathematical equation (1) is used.(Molecular weight)÷(Number of hydroxy groups)<40  Equation (1)
The literatures mention, as specific examples of the organic hydroxy compound, saccharides such as glucose, fructose, mannose, and ribose, sugar alcohols such as sorbitol, mannitol, and maltitol, and polyhydric alcohols such as glycerin. All of the sugars mentioned in the literatures belong to reducing sugars. However, the blending with each of those reducing sugars has little effects of suppressing the time-dependent decrease in viscosity in the alginic acid-containing aqueous composition.